1. Field of the Invention
The present invention relates to a substrate for a liquid crystal display used in a display section of an electronic apparatus, a liquid crystal display having the substrate, and a method of driving the display.
2. Description of the Related Art
Recently, liquid crystal displays have been put in use as monitoring device of television receivers and personal computers. In such applications, high viewing angle characteristics must be achieved to allow a display screen to be viewed in all directions. FIG. 20 is a graph showing transmittance characteristics relative to applied voltages (T-V characteristics) of a VA (Vertically Aligned) mode liquid crystal display. The abscissa axis represents voltages (V) applied to a liquid crystal layer, and the ordinate axis represents light transmittance. The line A indicates T-V characteristics in a direction perpendicular to the display screen (hereinafter referred to as “a square direction”), and the line B indicates T-V characteristics in a direction at an azimuth angle of 90° and a polar angle of 60° with respect to the display screen (hereinafter referred to as “an oblique direction”). An azimuth angle is an angle measured counterclockwise with reference to the direction toward the right of the display screen. A polar angle is an angle to a line that is vertical to the center of the display screen.
As shown in FIG. 20, there is a distortion in transition of transmittance (luminance) in the vicinity of the region enclosed by the circle C. For example, while transmittance in the oblique direction is higher than transmittance in the square direction for a relatively low gradation at an applied voltage of about 2.5 V, the transmittance in the oblique direction is lower than the transmittance in the square direction for a relatively high gradient at an applied voltage of about 4.5 V. As a result, a luminance difference within an effective range of driving voltages becomes small when viewed in the oblique direction. This phenomenon appears in a most significant way as a color variation.
FIGS. 21A and 21B show how an image displayed on a display screen varies in its view. FIG. 21A shows the image as viewed in the direction square to the screen, and FIG. 21B shows the image as viewed in an oblique direction. As shown in FIGS. 21A and 21B, the color of the image appears more whitish when the display screen is viewed in the oblique direction than when viewed in the square direction.
FIGS. 22A to 22C show gradation histograms of three primary colors, i.e., red (R), green (G), and blue (B) in a reddish image. FIG. 22A shows the red gradation histogram. FIG. 22B shows the green gradation histogram. FIG. 22C shows the blue gradation histogram. The abscissa axes of FIGS. 22A to 22C represent gradations (256 gradations from 0 to 255), and the ordinate axes represent the rate of presence (%). As shown in FIGS. 22A to 22C, relatively high gradations of red and relatively low gradations of green and blue are present in high rates of presence. When such an image is displayed on a display screen of a VA mode liquid crystal display and is viewed in an oblique direction, the high gradations of red appear relatively darker, and the low gradations of green and blue appear relatively lighter. Since differences in luminance between the three primary colors thus become smaller, the image appears in a whitish color as a whole.
The above-described phenomenon similarly occurs in a liquid crystal display in the TN (Twisted Nematic) mode that is a driving mode according to the related art. Patent Documents 1 to 3 disclose techniques for mitigating the above-described problem in a TN mode liquid crystal display. FIG. 23 shows a basic configuration of one pixel of a liquid crystal display which is based on those known techniques. FIG. 24 shows a sectional configuration of the liquid crystal display taken along the line X-X in FIG. 23, and FIG. 25 shows an equivalent circuit of the one pixel of the liquid crystal display. As shown in FIGS. 23 to 25, the liquid crystal display has a thin film transistor (TFT) substrate 102, an opposite substrate 104, and a liquid crystal layer 106 sealed between the substrates 102 and 104.
The TFT substrate 102 has a plurality of gate bus lines 112 formed on a glass substrate 110 and a plurality of drain bus lines 114 formed across the gate bus lines 112 with an insulation film 130 interposed between them. TFTs 120 are disposed in the vicinity of intersections between the gate bus lines 112 and the drain bus lines 114, the TFT 120 being formed as a switching element at each pixel. A part of a gate bus line 112 serves as a gate electrode of a TFT 120, and a drain electrode 121 of a TFT 120 is electrically connected to a drain bus line 114. Storage capacitor bus lines 118 are formed so as to extend in parallel with the gate bus lines 112 across pixel regions which are defined by the gate bus lines 112 and the drain bus lines 114. A storage capacitor electrode 119 is formed at each pixel above the storage capacitor bus lines 118 with the insulation film 130 interposed between them. A storage capacitor bus line 119 is electrically connected to a source electrode 122 of a TFT 120 through a control electrode 125. A storage capacity Cs is formed between a storage capacitor bus line 118 and a storage capacitor electrode 119.
A pixel region defined by gate bus lines 112 and drain bus lines 114 is divided into a sub-pixel A and a sub-pixel B. A pixel electrode 116 is formed at the sub-pixel A, and a pixel electrode 117 is formed at the sub-pixel B separately from the pixel electrode 116. The pixel electrode 116 is electrically connected to the storage capacitor electrode 119 and the source electrode 122 of the TFT 120 through a contact hole 124. The pixel electrode 117 is electrically floating. The pixel electrode 117 has a region which overlaps the control electrode 125 with a protective film 132 interposed between them, and the electrode is indirectly connected to the source electrode 122 as a result of capacitive coupling through a control capacity Cc formed in that region.
An opposite substrate 104 has a color filter (CF) resin layer 140 formed on a glass substrate 111 and a common electrode 142 formed on the CF resin layer 140. A liquid crystal capacity Clc1 is formed between the pixel electrode 116 at the sub-pixel A and the common electrode 142, and a liquid crystal capacity Clc2 is formed between the pixel electrode 117 at the sub-pixel B and the common electrode 142. Alignment films 136 and 137 are formed at the interface between the TFT substrate 102 and the liquid crystal 106 and the interface between the opposite substrate 104 and the liquid crystal, respectively.
Let us now assume that the TFT 120 is turned on to apply a voltage to the pixel electrode 116 and that a voltage Vpx1 is applied to the liquid crystal layer at the sub-pixel A. Then, since the potential is divided according to the ratio between the liquid crystal capacity Clc2 and the control capacity Cc, a voltage different from that applied to the pixel electrode 116 is applied to the pixel electrode 117 at the sub-pixel B. A voltage Vpx2 applied to the liquid crystal layer at the sub-pixel B is given by:Vpx2=(Cc/(Clc2+Cc))×Vpx1An actual voltage ratio Vpx2/Vpx1 (=(Cc/(Clc2+Cc)) is an item designed based on display characteristics of the liquid crystal display, and it is idealistically set in the range from 0.6 to 0.8.
When one pixel includes sub-pixels A and B having threshold voltages different from each other as thus described, a distortion of T-V characteristics as shown in FIG. 20 is distributed between the sub-pixels A and B. It is therefore possible to suppress the phenomenon of whitish appearance of an image when viewed in an oblique direction and to thereby improve viewing angle characteristics. The technique will be hereinafter referred to as a capacitive coupling HT (halftone grayscale) method.
While the above-described technique is disclosed in Patent Documents 1 to 3 on an assumption that the technique is used in a TN mode liquid crystal display, the technique is more advantageous when used in liquid crystal displays of the VA mode which has recently become the main stream in place of the TN mode.
FIGS. 26A to 26D are illustrations for explaining sticking that occurs in a liquid crystal display according to the related art employing the capacitive coupling HT method. FIG. 26A shows a black-and-white checker pattern displayed on a screen during a sticking test. Referring to the sticking test, a halftone of the same gradation is displayed throughout the screen immediately after continuously displaying the checker pattern shown in FIG. 26A for a certain time (e.g., 48 hours), and it is checked whether the checker pattern is visually perceived or not. When the checker pattern is visually perceived, the luminance of the screen is measured along the checker pattern to calculate a sticking rate. Let us assume that a represents the luminance of low-luminance regions of the checker pattern that is visually perceived and that the luminance of high-luminance regions is represented by a+b. Then, the sticking rate is defined as b/a.
FIG. 26B shows the display of the halftone on a screen of a liquid crystal display which does not employ the capacitive coupling HT method. FIG. 26C shows the display of the halftone on a screen of a liquid crystal display employing the capacitive coupling HT method. As shown in FIG. 26B, substantially none of the checker pattern was visually perceived when the halftone was displayed on the liquid crystal display which did not employ the capacitive coupling HT method. Luminance was measured along the line Y-Y′ in FIG. 26B, and there was a luminance distribution as indicated by the line c in FIG. 26D. The sticking rate was only 0 to 5%. On the contrary, the checker pattern was visually perceived as shown in FIG. 26C on the liquid crystal display employing the capacitive coupling HT method. Luminance was measured along the line Y-Y′ in FIG. 26C, and there was a luminance distribution as indicated by the line d in FIG. 26D. The sticking rate was 10% or more. As thus described, substantially no sticking occurred on the liquid crystal display which did not employ the capacitive coupling HT method, whereas the liquid crystal display employing the capacitive coupling HT method had a problem in that relatively dense sticking occurred.
As a result of evaluation and analysis on a characteristic distribution in a pixel of the liquid crystal display in which sticking occurred, it was revealed that the sticking occurred at the sub-pixel B having the pixel electrode 117 which was electrically floating. The pixel electrode 117 is connected to the control electrode 125 through a silicon nitride film (SiN film) having a very high electrical resistance and is connected to the common electrode 142 through the liquid crystal layer which also has a very high electrical resistance. Therefore, an electrical charge in the pixel electrode 117 is not easily discharged once it is charged. An electrical potential for each frame is written in the pixel electrode 116 at the sub-pixel A that is electrically connected to the source electrode 122 of the TFT 120, and the pixel electrode 116 is connected to the drain bus line 114 through an active semiconductor layer of the TFT 120 which is very much lower in electrical resistance than the SiN film and the liquid crystal layer. Therefore, the electrical charge charged in the pixel electrode 117 will never become undischargeable.
FIG. 27A is a graph showing changes in a capacity ratio, a voltage ratio, and a liquid crystal dielectric constant ∈ of the liquid crystal display according to the related art employing the capacitive coupling HT method. The abscissa axis of FIG. 27A represents voltages (V) applied to the liquid crystal layer at sub-pixel A, and the ordinate axis represents the capacity ratio, voltage ratio, and dielectric constant. The line e indicates a capacity ratio Cc/Clc2 between the control capacity Cc and the liquid crystal capacity Clc2. The line f indicates a voltage ratio Vpx2/Vpx1 between the voltage Vpx1 applied to the liquid crystal layer at the sub-pixel A and the voltage Vpx2 applied to the liquid crystal layer at the sub-pixel B. The line g indicates the dielectric constant ∈ of a negative liquid crystal that is used in the VA mode liquid crystal display. FIG. 27B shows changes in the voltage ratio Vpx2/Vpx1 with the ordinate axis (voltage ratio) drawn on an enlarged scale.
As shown in FIGS. 27A and 27B, in the liquid crystal display according to the related art employing the capacitive coupling HT method, the voltage ratio Vpx2/Vpx1 decreases as the applied voltage increases. In a liquid crystal display according to the related art employing the capacitive coupling HT method, about 50 to 80% of a pixel as a whole is occupied by the sub-pixel B which is lower in luminance than the sub-pixel A because a lower voltage is applied to the liquid crystal layer thereof. For this reason, a high transmittance can not be achieved at the sub-pixel B even when a voltage (5.5 to 7 V) for displaying white is applied to the pixel electrode 116. As a result, the luminance of the entire pixel is as low as about 40 to 80% of that of a liquid crystal display which does not employ the capacitive coupling HT method. A distortion of luminance transition occurs in the region of low gradations and medium gradations as shown in FIG. 20. It is therefore idealistic that a difference between threshold voltages of sub-pixels A and B is greater when the voltages are low and is smaller when the voltages are high. An idealistic liquid crystal display which is excellent in both of viewing angle characteristics and luminance can be provided, for example, by applying a voltage Vpx2 as lows as 1.5 to 2 V to the liquid crystal layer at the sub-pixels B when the voltage Vpx1 applied to the liquid crystal layer at the sub-pixel A is 2.5 V (that is, there is a great voltage difference (Vpx1−Vpx2)) and by applying a voltage Vpx2 as high as 5 to 5.5 V to the liquid crystal layer at the sub-pixels B when the voltage Vpx1 applied to the liquid crystal layer at the sub-pixels A is 5.5 V (that is, there is a small voltage difference (Vpx1−Vpx2)). However, in a liquid crystal display having a configuration in which the control capacity Cc and the liquid crystal capacity Clc2 are connected in series as shown in FIGS. 23 to 25, the voltage ratio Vpx2/Vpx1 is determined by the capacity ratio Cc/(Clc2+Cc). When the capacity ratio Cc/(Clc2+Cc) is constant, the voltage ratio Vpx2/Vpx1 is constant. Then, on the contrary to the above-described idealistic case, the voltage difference (Vpx1−Vpx2) will be greater, the higher the voltages are.
The above-described problem is made more significant by fluctuations of the liquid crystal capacity Clc2. As will be apparent from the line g shown in FIG. 27A, the dielectric constant ∈ of the liquid crystal is greater, the higher the voltage applied thereto. Since the liquid crystal capacity Clc2 increases with the dielectric constant ∈, the capacity ratio Cc/Clc2 becomes smaller, and the voltage ratio Vpx2/Vpx1 determined by Cc/(Clc2+Cc) also becomes small. As shown in FIG. 27B, the voltage ratio Vpx2/Vpx1 is 0.72 at low voltages of about 0 to 2 V, and the voltage ratio Vpx2/Vpx1 becomes as small as about 0.62 at a voltage of 5 V for displaying white. That is, the voltage difference (Vpx1−Vpx2) becomes greater at high voltages. Therefore, liquid crystal displays according to the related art employing the capacitive coupling HT method have a problem in that it is difficult to obtain high luminance.
The relationship between the liquid crystal capacity and the voltage ratio can result in more serious display irregularities when there is variation of the cell thickness. The transmittance of a liquid crystal display panel is determined by the retardation of the liquid crystal layer and, in general, the transmittance increases with the cell thickness and decreases with the cell thickness. In the case of a liquid crystal display according to the related art employing the capacitive coupling HT method, the liquid crystal capacity Clc2 decreases as the cell thickness increases, which results in an increase in the luminance of the pixel as a whole because the voltage ratio Vpx2/Vpx1 approaches 1. On the contrary, the liquid crystal capacity Clc2 increases as the cell thickness decreases, which results in a reduction in the luminance of the pixel as a whole because the voltage ratio Vpx2/Vpx1 approaches 0. That is, the liquid crystal display according to the related art employing the capacitive coupling HT method has a problem in that display irregularities are highly visible because fluctuation of the cell thickness results in a synergetic effect between variation of transmittance attributable to variation of the retardation and variation of transmittance attributable to variation of the voltage ratio.
Patent Document 1: JP-A-2-12
Patent Document 2: U.S. Pat. No. 4,840,460
Patent Document 3: Japanese Patent No. 3076938
Patent Document 4: JP-A-8-146464
Patent Document 5: JP-A-2001-235766